1. Field of the Invention
This invention relates to compression systems, and more particularly to fluid refrigerant compression systems utilizing multiple compression stages, including means for recycling a fraction of the refrigerant after a first partial expansion thereof.
2. Prior Art
Multiple stage refrigeration compression systems are known in the art. Gygax U.S. Pat. No. 2,497,450, for example, provides a pair of compressing pistons and cylinders, one operating at low pressure and providing compressed gas to a coil, or intercooler, within a condenser, and the other receiving the discharge of the intercooler and providing high pressure gas to the condensing coil. A refrigerant storage tank receives the condensed liquid refrigerant and supplies the same through an intermediate expansion valve to an evaporator. The apparatus does not provide for removal of intermediate pressure gas to achieve savings in energy consumption associated with repeated, unnecessary compression and expansion thereof.
Anderson U.S. Pat. No. 4,173,433 discloses a compressor including a double acting piston to provide two stage operation. In order to avoid unnecessary compression of gas in the piston cavity, vents are provided from the cavity to the low pressure intake. The disclosed compressor provides no structure for utilizing a vapor fraction of partially expanded refrigeration fluid, however, nor is recompression of flash gas considered.
The use of flash gas from an intermediate pressure flash tank in a mixture with discharge from a first stage compressor to provide the input for the second stage compressor is described in "Principles of Refrigeration", Second Edition, R. J. Dossat, John Wiley and Sons, Inc., New York, N.Y. 1978. There is no provision, however, for separately compressing the flash gas prior to mixing with the high pressure refrigerant. See pages 525-536, for example.
A number of references disclose multiple stage compression for refrigeration systems. Terry U.S. Pat. No. 2,031,237 and Newton U.S. Pat. No. 2,461,760 are illustrative of prior art usage of essentially parallel compression stages, rather than sequential compressors.
Rayburn U.S. Pat. No. 2,719,409 discloses a two-refrigerant apparatus including a pair of heat absorbers. Christensen U.S. Pat. No. 880,125 provides an early compound air compressor, including a crank shaft driven piston rod to which are secured two pistons for suction and compression of a gas in two cylinders separated by a valve head/cylinder head.
None of the references provide an apparatus for metering intermediate pressure fluid to a compression chamber therefor and for mixing the output of one of the main compression stages with the repressurized intermediate pressure fluid. The prior art fails to insulate a flash tank from effects of changes in the refrigeration load, exposing the tank to such effects by mixing the gas from a lower pressure stage compressor with the flash gas prior to compression to a higher pressure. Such resulting variation in the flash tank pressure causes changes in the pressure drop across a second-stage expansion valve used in conjunction with the refrigerating evaporator and heat absorbing coil, leading to requirements for increased expansion valve size and thus a reduced degree of control over the flow of refrigerant.